1. Field of the Invention
The present invention relates generally to a phase modulated servo positioning system in a direct access storage device (DASD) and more particularly to a phase modulated servo method and apparatus for use with transducer heads having instabilities.
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks.
All DASD units must have a method to position each data head over the proper radial location to write a track and again, to position it very close to the same location to read the track. With the higher level files using a voice coil type of actuator, a feedback mechanism must be provided to locate and stably hold the head on a given track. Typically, track accessing and track following are provided utilizing a magnetically written pattern in the DASD unit. A dedicated servo system employs one surface of one of the disks in the DASD on which to have all the tracking and access information. A sector servo system uses small portions of tracks between each or between several sectors on each track of each data surface to provide the tracking and access information. A hybrid servo system uses both to obtain advantages of each type of servo.
Magneto-resistive (MR) heads are a great advancement in read/write technology for DASD; however, the MR head""s worst characteristics is head instability. The term head instability is used to describe a number of readback waveform abnormalities. Although the exact cause of head instability is not known, the most popular model states that head instability is due to non-linear changes in the transfer curve of the MR element. These jumps may be due to non-linear changes in magnetic domain movement in the MR element. The jumps in the transfer curve cause corresponding jumps in the readback waveform. Most importantly, the non-linear features of the element transfer curve change each time the head is excited. The excitation typically is due to writing data, but can also be due to landing the MR head on the disk, hitting an asperity on the disk or changing MR bias current.
A problem exists to generate a position error signal (PES) from a phase modulated (PM) servo pattern in the presence of magnetic recording head instability. When instabilities are present in the readback signal, a PES generated from a phase modulated (PM) servo signal can result in a track misregistration (TMR) error.
It is a principal object of the present invention to provide a phase modulated servo method and apparatus for use with transducer heads having instabilities that overcomes many of the disadvantages of prior art arrangements.
In brief, the objects and advantages of the present invention are achieved by a phase modulated servo method and apparatus used in a disk file. The disk file includes at least one disk mounted for rotation about an axis and the disk has at least one disk surface for storing data. At a predefined location of the disk surface a plurality of servo tracks of a predetermined high gain servo pattern are written. The predetermined high gain servo pattern includes 360xc2x0 phase difference information within each data cylinder. The servo tracks are detected for identifying servo phase information.